Modern Quality Amplifier Conceptual Outline

May is a very busy month for me personally so I have only a short update this week. My last post was about the Quality Amplifier, a forerunner to the well-known Williamson amp. I proposed a modernized 6V6 A2 version based on the same topology that could probably crack double digit power.

In the conceptual outline above we have a MOSFET rather than a tube as a concertina splitter to save heater power. The unity gain MOSFET splitter feeds a tube differential pair to generate enough voltage gain to drive the outputs. MOSFET grid drivers with a CCS load help the differential stage cope with the drastic impedance changes at the output tube grids in A2 operation. Outputs are wired as triodes, of course.

The A2 drivers will go on a PCB. I also have one designed for the MOSFET splitter, though that’s simple enough to wire any way you might want. We’d need just three tubes per channel: a pair of 6V6s and a dual triode driver. My driver pick at the moment is probably the 5965 (or a pin-compatible 12AT7 if it needs to be new production).

I’m also happy to report that progress is being made on the preamp project. Hopefully I’ll have pictures to start sharing in the very near future!

Direct coupled stages

If you spend enough time haunting DIY tube amp websites and books, you will inevitably come across the theme of direct-coupled tube circuits. N-type and p-type transistor sandwiches make direct coupled circuits almost trivial. Tubes, which are “n-type” only, are not quite so simple to marry anode to grid. And yet the siren sings, drawing in the adventurous tube spirits.

“Why should we want to direct couple in the first place?” you ask, your socratic gland tingling. Many solid state amplifiers take advantage of the direct coupling to increase the levels of negative feedback. With tubes, we’re often more interested in maximizing the inherent linearity of triodes in open loop Class A amplification (but for a good counter example, see Jones’s Crystal Palace in Valve Amplifiers 4th ed).

Ostensibly, eliminating a coupling capacitor or transformer leaves less in the signal path between input and output, making whatever you are building more transparent (if you consider caps to be a significant source of coloration). Eliminating a coupling capacitor also removes a potential source for blocking distortion (if you are prone to driving amps to clipping, though a cap-bypassed cathode resistor can still cause you problems). In my opinion, the most compelling reason to direct couple is that it makes A2 (positive grid bias) operation a possibility.

Following are some (mostly untested) scratch-pad ideas and notes for “simple” direct-coupled SET amplifiers.

Fig 1: Simply using an abnormally large cathode resistor under the output tube raises its cathode above the anode voltage of the driving stage. This dissipates a lot of extra power in the output section and doesn’t really contribute anything to A2 operation. Still, a fun party trick.

resistor load dc

Fig 2: Using a resistor divider to lower the dc voltage seen by the output tube’s grid. This reduces the gain of the first stage and probably still requires you raise the cathode of the output stage (see Jones for good reading on this).

level shift dc

Fig 3: The Free Lunch style of choke loading the driving stage is as nifty as it is temperamental (in my experience). You are still dissipating power in the cathode of the output tube. See also Loftin-White variations discussed at TubeCAD.

choke loaded dc

Fig 4: Currently simmering on my back burner, a MOSFET gyrator sets a reliable voltage on the grid of the output tube and its low output impedance enables A2 operation. Rather than raising the cathode by dissipating power in a bias resistor, the cathode is raised by a separate power supply (must be rock solid). Additional stacked supplies provide B+ for the output tube and driving stage.

mosfet dc

It should be pointed out that direct coupling will almost always require some extra calculating, measuring, and adjusting of whatever you build (you get a glimpse of this with the El Estudiante cathode resistor trial and error). You’re also likely to pigeon hole a direct coupled circuit to very specific tubes, not to mention bias points (which must be maintained). But despite these warnings, once you’ve heard the legend of the circuit without caps, it may already be too late.

 

 

 

Something for beginners

Pete Millett’s Starving Student was one of the first amps I ever built completely from scratch. Unfortunately, the 19J6 tubes have become rare (or at least no longer dirt cheap) due to all the bright eyed DIYers scooping them up to build amps. I think the world needs another <50V tube amp for beginners, so I’m designing one. Like the original, it’s an oddball tube with a MOSFET buffer and an off-the-shelf power brick (same brick, in fact).

Millett is one of my personal tube heroes. This is a tribute.  Full write up coming soon (and parts values subject to change once tested).